It is common in the electronic computer field to provide for one central processing unit a plurality of magnetic disc units to thus provide a large external memory capacity. In such cases, a magnetic disc controller is provided between the magnetic disc units and an input-output processing device for controlling the magnetic disc units.
The operation of the magnetic disc controller is as follows. Referring to FIG. 1, when a central processing unit 1 is to transfer information stored in a first disc unit D1 to a main memory 2, the CPU 1 first actuates the input-output controller 3. Then the input-output controller transmits an instruction to the magnetic disc unit controller 4. In response to this instruction, the disc unit controller 4 selects the first disc unit D1 among the plural disc units D1 to Dn. Next, the positioning information is transmitted to the disc unit controller 4 from the input-output controller 3 to select a specific track of the disc unit D1 in which is stored the desired information. In accordance with the positioning information received from the input-output controller 3, the disc controller 4 sends a positioning instruction to the disc unit D1. After the positioning is completed, and when an interrupt indicating completion of the positioning operation is issued, an address field of a desired record is read from the track selected by the positioning operation.
It is usual that a plurality of records are present on one track. If a track is a defective track, the address field of one of the records of that track contains information indicating that the track is defective. When the information indicating a defective track is detected, an address of a substitute track is provided. In accordance with the substitute track address, positioning to the substituted track is performed. After a positioning completion interrupt has been received and the address field of the substitute track has been read, the data field following the substitute track address is read out. The read-out data is transferred through the input-output controller 3 to the main memory 2, sequentially. If it is decided during the defective track discrimination process that the track is a normal track, the data field following the track address is read out immediately.
The above-described positioning control system has some significant disadvantages. Specifically, since when the computer attempts to read or write a particular record at a designated address, it is not known whether the track location where the record is to be read from or written into is a defective location. Thus, the disc unit controller 4 has to (1) confirm whether or not the assigned track is defective, which takes about 30-100 ms, and, if so, (2) read out the substitute track address and then move to the indicated substitute track position, which takes about 10-20 ms. The extra time required to perform the two operations is very long compared with the time required to read out a data field in a good track, which takes only several milliseconds on the average. Especially, if a defective track is to be accessed frequently, the overhead time and processing time for accessing the desired information are very high.
Since the defect rate of tracks on a magnetic disc increases with the packing density of information stored on the disc, and as packing densities have been increasing due to increasing data storage requirements and improvements in magnetic disc operating systems, the above-described system has become particularly disadvantageous.